When to use STL bitsets instead of separate variables?

Question

In what situation would it be more appropriate for me to use a bitset (STL container) to manage a set of flags rather than having them declared as a number of separate (bool) variables?

Will I get a significant performance gain if I used a bitset for 50 flags rather than using 50 separate bool variables?

Answer 1

Well, 50 bools as a bitset will take 7 bytes, while 50 bools as bools will take 50 bytes. These days that's not really a big deal, so using bools is probably fine.

However, one place a bitset might be useful is if you need to pass those bools around a lot, especially if you need to return the set from a function. Using a bitset you have less data that has to be moved around on the stack for returns. Then again, you could just use refs instead and have even less data to pass around. :)

Answer 2

std::bitset will give you extra points when you need to serialize / deserialize it. You can just write it to a stream or read from a stream with it. But certainly, the separate bools are going to be faster. They are optimized for this kind of use after all, while a bitset is optimized for space, and has still function calls involved. It will never be faster than separate bools.

Bitset

• Very space efficient
• Less efficient due to bit fiddling
• Provides serialize / de-serialize with op<< and op>>
• All bits packed together: You will have the flags at one place.

Separate bools

• Very fast
• Bools are not packed together. They will be members somewhere.

Decide on the facts. I, personally, would use std::bitset for some not-performance critical, and would use bools if I either have only a few bools (and thus it's quite overview-able), or if I need the extra performance.